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Why do T cells recognize infections that are inside body cells?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

T cells are unable to bind to free-floating antigens, which allows pathogens like viruses to evade circulating antibodies once they have entered a body cell. To counter this, the immune system utilizes the major histocompatibility complex (MHC) to display evidence of infection on the cell surface.12

When a cell is infected, it partially digests the invader and moves fragments of the foreign protein to its surface. These fragments are bound to MHC molecules, which act as a display platform. T cells possess specific T-cell antigen receptors that recognize and bind to these foreign fragments only when they are associated with MHC molecules.12

This process allows cytotoxic T cells to identify and destroy infected cells, effectively targeting pathogens that are otherwise hidden from the rest of the immune system. Accessory receptors, such as CD4 or CD8, further strengthen the bond between the T cell and the target cell to ensure a precise and effective immune response.13